1. Field of the Invention
The present invention relates to wireless local network communication, and more particularly to an 802.11g wireless network.
2. Description of the Related Art
The conventional wireless LAN standard includes three physical layer standards: IEEE 802.11a, 802.11b, and 802.11g. The IEEE 802.11a standard adopts orthogonal frequency division multiplexing (OFDM) modulation to support a maximum data transfer rate of 54 Mbps. The IEEE 802.11b standard adopts the direct sequence spread spectrum (DSSS) technique and complementary code keying (CCK) modulation to support a maximum data transfer rate of 11 Mbps. Because IEEE 802.11a and IEEE 802.11b devices cannot work together in a single wireless network, the IEEE 802.11g standard is provided to accommodate both the higher data rate OFDM modulation and the lower data rate DSSS modulation. Thus, although an 802.11g device can support a high data rate, it can still communicate with an 802.11b device, and an 802.11g network can comprise both 802.11b devices and 802.11g devices at the same time.
An 802.11g access point is the core of an 802.11g network. All other devices in the wireless network connect to the external network or a backbone network through the 802.11g access point. When an 802.11g wireless network contains both 802.11b devices and 802.11g devices, the 802.11g devices can understand DSSS signals emitted from 802.11b devices, and will wait until the DSSS signal ends to communicate with the access point. 802.11b devices, however, do not understand OFDM signals emitted from 802.11g devices, and will treat the OFDM signal as noises. If an 802.11b device attempts to communicate with the access point while an 802.11g device is transmitting OFDM signals, the 802.11b device will not wait to emit a DSSS signal and will interfere with the OFDM signals of the 802.11g device. Thus, the access point or the 802.11g devices must notify the 802.11b devices to suppress their communication before the access point of the 802.11g devices emit OFDM signals.
FIG. 1 shows the down link procedures initiated by an access point 102 in an 802.11g network 100. The 802.11g network 100 includes three elements: an 802.11g access point 102, an 802.11g station 104, and an 802.11b station 106. When the access point 102 wants to initiate communication with the 802.11g station 104, it must send a 802.11b request to send (RTS) command 112 modulated with the DSSS and CCK method to the 802.11b station 106. After receiving the RTS command 112, the 802.11b station 106 will return a 802.11b clear to send (CTS) command 114 to the access point 102, indicating that the 802.11b station 106 will not emit DSSS signal for a period of time requested by the RTS command 112. The access point 102 then issues an 802.11g RTS command 116 to the 802.11g station 104. After receiving the RTS command 116, the 802.11g station then returns a 802.11g CTS command 118 to the access point 102, indicating that the 802.11g station 104 is ready to receive signals from the access point 102. The access point 102 then begins sending OFDM signals to the 802.11g station 104 for transferring data.
FIG. 2 shows the up link procedures initiated by the 802.11g station 104 in the 802.11g network 100. When the 802.11g station 104 wants to initiate communication with the access point 102, it must send an 802.11b CTS to self command 212 modulated with the DSSS/CCK method to the 802.11b station 106 to request that the 802.11b station 106 not emit DSSS signals for a period. The 802.11g station 104 then issues an 802.11g RTS command 214 to the access point 102. After receiving the RTS command 214, the access point 102 then return a 802.11g CTS command 216 to the 802.11g station 104, indicating that the access point 102 is ready to receive signals from the 802.11g station 104. The 802.11g station 104 then begins sending OFDM signals to the access point 102 for transferring data.
Transmitting RTS, CTS, and CTS to self commands, however, requires additional bandwidth, reducing the bandwidth efficiency of the wireless network 100. In addition, some applications do not require bandwidth as high as 54 Mbps to support communication. For example, a VOIP call only requires 2 Mbps of bandwidth to support the voice packet traffic. If the access point and the 802.11g station use the OFDM modulation method to support the VOIP application, both the bandwidth and the power of the 802.11g station will be wasted. If the 802.11g station is a handheld device, the battery power is limited and the 802.11g station will only operate for a certain amount of time. Moreover, although the data rate of OFDM modulation is higher than the data rate of DSSS modulation, but, DSSS modulation has a wider transmission range than OFDM modulation. Thus, OFDM modulation technique still has deficiencies in an 802.11g network.